Gases & the Ideal Gas Law
A gas is chaos with rules. Four knobs — pressure, volume, temperature, amount — locked together by one equation.
Builds on: 3.1 Solid, Liquid, Gas2.2 The Mole: Chemistry's Dozen
Pressure is drumming
A gas particle hits the container wall and bounces off, giving the wall a tiny shove. Multiply by 10²³ collisions per second and the drumming blurs into a steady push: pressure. This picture makes gas behaviour almost obvious. Shrink the volume → particles hit walls more often → pressure rises. Heat the gas → particles hit harder and more often → pressure rises. Add more gas → more drummers → pressure rises.
One law to rule the knobs
Everything above is packed into this one line, with R ≈ 8.314 J/(mol·K) as the conversion constant. The non-negotiable detail: T must be in kelvin (K = °C + 273.15). Celsius has its zero at an arbitrary point — water’s freezing — while gas physics cares about absolute motion. At 0 K motion stops; doubling kelvin genuinely doubles the pressure. Doubling “degrees Celsius” means nothing.
A famous consequence: at room conditions, one mole of any ideal gas fills about 24 litres — hydrogen, oxygen, CO₂ alike. The identity of the particles barely matters when they spend their lives far apart.
Special cases you already know
- Squeeze at constant T (Boyle): halve V → double P. Syringes, pistons, diving.
- Heat at constant V (Gay-Lussac): P rises with T. Why aerosol cans say “never throw into fire”.
- Heat at constant P (Charles): V grows with T. Why a hot-air balloon rises and a sealed bag puffs up in the sun.
The law assumes point-particles with no attractions — nearly true for ordinary gases at ordinary conditions. Squeeze hard or cool near condensation and the intermolecular forces from the last lesson reappear; that’s exactly when real gases start to liquefy and the law bends.
⚗️ Lab — The Piston Machine
A gas in a cylinder with three knobs and one pressure gauge.
- Halve the volume at fixed T — watch P double as the wall-drumming doubles.
- Heat from 300 K to 600 K at fixed volume: the particles blush red and P doubles.
- Find settings where the gauge reads ≈ 101 kPa — that’s the air around you.
Work it by hand — 0 / 4
No multiple choice here: compute the value and type it. Suffixes like 2.5k, 20m or 100µ are understood; answers within ±2% count.